1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kinematics elevator problem

  1. Mar 31, 2015 #1
    1. The problem statement, all variables and given/known data
    A boy standing in an elevator accelerating upward throws a ball upward with a vo.
    The ball returns to his hands after time t. The acceleration of the lift is


    2. Relevant equations


    3. The attempt at a solution
    After time t, displacement of the ball = displacement of the lift
    vo t - 1/2 gt2 = ?
    What should the displacement of the lift be?
    According to me, it should be ut + 1/2at2.
    Problem is, I don't know what the initial velocity of the lift is, i.e. what u is.
    What do I do?
     
  2. jcsd
  3. Mar 31, 2015 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    The quantity v0 is probably to be interpreted as the initial velocity of the ball relative to the elevator. You are writing equations relative to the ground. So, what is the initial velocity of the ball relative to the ground?
     
  4. Mar 31, 2015 #3

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You are likely expected to understand that the velocity v0 is the velocity of the ball relative to the elevator. You can make any assumption on the velocity of the elevator, it should not affect the result - simply assume it is an undetermined velocity u.
     
  5. Mar 31, 2015 #4
    Ok thank you!
    So, displacement of the body in the frame of the elevator is 0 in time t, and so I can consider a pseudo force a on the elevator.
    v0t - 1/2 (g+a) t^2 = 0
    a= (2v0 - gt)/t
     
  6. Mar 31, 2015 #5

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Correct.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Kinematics elevator problem
  1. Elevator kinematic (Replies: 10)

  2. Elevator problem (Replies: 7)

  3. Elevator Problem (Replies: 5)

Loading...